206 RECENT PROGRESS OF THE THEORY OF VISION. 



and thus acquired the character of an unusually careful 

 observer. For this phenomenon was anything but ob- 

 vious ; it can only be seen by strong side illumination, 

 in darkness otherwise complete, only when the observer 

 takes a certain position, and then all he sees is a faint 

 misty reflexion. But this faint reflexion was destined 

 to become a shining light in a dark corner of science. It 

 was in fact the first appearance observed in the living 

 eye which came directly from the lens. Sanson imme- 

 diately applied his discovery to ascertain whether the 

 lens was in its place in cases of impaired vision. Max 

 Langenbeck made the next step by observing that the 

 reflexions from the lens alter during accommodation. 

 These alterations were employed by Cramer of Utrecht, 

 and also independently by the present writer, to arrive 

 at an exact knowledge of all the changes which the lens 

 undergoes during the process of accommodation. I suc- 

 ceeded in applying to the inoveable eye in a modified 

 form the principle of the heliometer, an instrument by 

 which astronomers are able so accurately to measure small 

 distances between stars in spite of their constant apparent 

 motion in the heavens, that they can thus sound the 

 depths of the region of the fixed stars. An instrument con- 

 structed for the purpose, the ophthalmometer, enables 

 us to measure in the living eye the curvature of the 

 cornea, and of the two surfaces of the lens, the distance 

 of these from each other, &c., with greater precision 

 than could before be done even after death. By this 

 means we can ascertain the entire range of the changes 

 of the optical apparatus of the eye so far as it affects 

 accommodation. 



The physiological problem was therefore solved. Ocu- 

 lists, and especially Donders, next investigated the indi- 

 vidual defects of accommodation which give rise to the 

 conditions known as long sight and short sight. It was 



